| A trend in inorganic instrumental analytical chemistry has been the development of microscopic equivalents of established analytical techniques. Usually, these micro beam methods are able to analyze (microscopically) small parts of a large solid specimen. The fact that the techniques yield information on trace element concentrations in well-defined locations on the surface of a material makes them very powerful investigation techniques for heterogeneous materials of industrial, geological, biological or environmental nature. When such methods need to be used to determine local trace element concentrations in a reliable and traceable manner, appropriate reference materials (RM) must be used to calibrate the instruments. Micro-homogeneous materials with certified levels of trace element abundance are rare.; In this work, a number of standard reference materials from NIST, BCR-IRMM and USGS are examined by means of the non-destructive synchrotron microscopic X-ray fluorescence analysis (μ-XRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A procedure based on a large number of repeated analyses of small absolute amounts of the RMs allows to calculate the minimal representative mass of the standard. The micro-heterogeneity of these RMs is investigated with the aim of evaluating their suitability as reference materials for trace-level micro analytical techniques.; A Monte Carlo simulation model is constructed for both homogeneous and heterogeneous materials to elucidate the dependence of the calculated minimal representative mass on the total analyzed mass in case of materials showing strongly micro-heterogeneous features. Results of the model calculations are compared to experimental data obtained from a homogeneous reference material NIST SRM 613 and a more heterogeneous material NIST SRM 1577a. Also the effect of nuggets and islands in the sample is evaluated by means of these Monte Carlo simulation models. From the simulation results, one can conclude that the procedure is able to calculate the minimal representative mass of both homogenous and heterogeneous reference materials, provided a sufficient amount of material is analyzed.; When analyzing the micro-heterogeneity of a number of existing reference materials of NIST and IRMM-BCR, one can conclude that the heterogeneity is different for each element in the sample, even up to the fact that some elements show a homogeneous response when employing a mass that is considerably lower than the minimal representative mass indicated on the certificate. |
